Most of AC induction motors type. They are generally easier and cheaper to build than similar machines of a direct current. They do not have the switch, slip rings, or brush, and there is no electrical connection to the rotors. Only the stator winding is connected to AC power, and, then, as follows from their name, produces induction currents in the rotor. General and especially simple form rotor for this type of engine-squirrel-cage rotor (see Fig. 7-1). It was so named because of its similarity to a treadmill type " squirrel wheel. Asynchronous electric motor on the basis of a rotating magnetic field. This is achieved with the help of several field of the stator windings (poles), each pair which is excited from the network of alternating current with voltage of the same amplitude and frequency, and phase IDPs from voltage supply in adjacent pairs. Fig. 7-2 shows how the magnetic field rotates in a four-pole induction motor where the voltage up to two pairs of poles 90 in phase with each other.
When the rotor is placed in statorР‚'s rotating field induced currents create your own fields, which react with statorР‚s field and click around the rotor.
Please note that some of the rotors are skewed. The tilt rotor refers to the angle between the conductor slots and end of the rotor lamination. As a rule, the conductors are in a nearly straight line, but for high torque rotor is skewed, which increases the angle of conductors. The term full tilt determines the maximum number of practical (see Fig. 7-3). Fig. 7-4 shows how the rotor is in reference to the stator and the end of the bells, holding it in place.
A number of types of AC motors. Types presented here are those most often encountered when working with heating, air conditioning, refrigeration equipment.
Shaded-pole asynchronous motor single-phase motor. He uses a unique method to start the rotor turning. The effect of a moving magnetic field is produced by building the stator in a special way (see Fig. 7-5)
Part of the pole part of the surface is surrounded by a copper belt called shading coil. Strap causes the field to move back and forth across the surface of the pole pieces In Fig. 7-6, numbered sequence of points of the magnetization curve shown. As the alternating field of the stator begins to grow from zero Fig. 7-6(1), lines of force expansion over the face of the pole piece and cut the strap. The voltage induced in the strap. The current that results generates a field that opposes the cutting action (and limits) of the main field. This action causes certain actions. As the field is increased from zero to 90, a large part of the magnetic field lines are concentrated in the unshaded part of the pole Fig. 7-6(1). In 90 region reaches its maximum value. Since the power line stopped expanding, and electromagnetic fields (EMFs)is induced in the belt, and not the contrary magnetic field is generated. As a result, the main field uniformly distributed across the poles, as shown in Fig. 7-6(2).
90 180, the main field begins to decrease, or crumbling inside. In the field generated in the strap opposes the collapse field. The effect is the concentration of power lines in the shady part of the poles, as shown in Fig. 7-6(3).
Please note that 0 180 main field has shifted over the pole person from unshaded the shaded part. 180 360main field passes through the same changes as it was from 0 180. However, it is now in the opposite direction Fig. 7-6(4). The direction of the field does not affect the way the shaded works. Movement in the field of the same, in the second half-hertz, as it was in the first half, Hertz.
Movement in the field and back into the shadows and unshaded portions produces a weak moment. This point is used to start the engine. Due to the low starting torque, shaded pole motors built only in small sizes. They go on devices such as fans, timers, and blowers...